Dynamic Package Loading/Importing

Question

Hi,
I'm developing an open framework for network admission control / network access protection which bases on independent modules handling different procedures like authentification, accounting and so on.
Right now I dynamically load my modules into my framework at startup or during runtime (hot-pluggable) using the well-known simple method:
className = "package.classname" // just an example
rawClass = Class.forName(className);
newModule = (AModule) rawClass.newInstance();
This works fine as long as
- The module which is going to be loaded only consists of one class and doesn't need other classes (e.g. for Dispatcher Threads ...) to run. This would still be no problem if I simply place all needed classes in the right directories, but I'd like to ship modules as packages for an easier handling of new modules.
- The module doesn't need any libraries like e.g. the mySQL-JDBC-Connector. Right at this point I run into serious troubles cause I've to specify these external jars before starting my framework (java -cp ...).
I haven't found any way to dynamically extend the classpath / to add whole jar's/packages during runtime yet so hopefully anyone could give me a hint how to implement that.
In short I'd like to implement the following functionality: Without being in classpath at startup a package should be accessible and all classes of the package should be loadable - more or less like being in classpath but without being specified to be in classpath at startup.
Thank you very much in advance
Bastian

Answer 1

Hi,
Like different classes that you created? Then you
put them in the same jar.
Even if you put them in seperate jars the original
jar has to get into the class path somehow. And
those other jars would get there the same way.For sure the framework itself is in one single JAR included in the classpath (or if extracted the class files lie in a path hierarchy included in the classpath).
Again the first class MUST be in your class path for
the code that you posted to work. I know that it has to be - but I'd like to avoid that. The code I've posted is the way it works right now. But I'd like to change that so that I don't need the first class to be in the class path.
That requires that you do something to get them
there.That's what I'm trying to find out - how can I get classes into the classpath after having started the runtime if they are bundled in external JARs (like e.g. the JDBC)?
How exactly are you currently delivering your stuff?I don't deliver any modules yet because I'd like to find a proper and easy way to handle that class/package loading problem before putting the project online.
My idea is:
- One JAR for the framework including all classes without modules.
- One JAR for every module which should be loaded and instantiated at runtime as defined in a global configuration file with the capability of "hot plugging", which means that I'm able load/start and stop/unload modules during runtime whenever I'd like to.
To implement the second part I guess I need a way to dynamically extend the classpath or dynamically load JARs.
There is no way to extend the class path. That is a
security feature of java.I see. What about the JarLoader I've found in the meanwhile written by Melanie Green. Is it a proper workaround?
You might note also that for the previous suggestion
it is complicated to load a jdbc driver via a
classloader. You can not use the standard code to do
it. There is a thread in the JDBC forum about this.I'll take a look - thank you very much!
Bastian